# std::partition_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt1, class OutputIt2, class UnaryPred > std::pair<OutputIt1, OutputIt2> partition_copy( InputIt first, InputIt last, OutputIt1 d_first_true, OutputIt2 d_first_false, UnaryPred p ); | (1) | (since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class UnaryPred > std::pair<ForwardIt2, ForwardIt3> partition_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first_true, ForwardIt3 d_first_false, UnaryPred p ); | (2) | (since C++17)

1) Copies the elements from the range `[`first`,` last`)` to two different ranges depending on the value returned by the predicate p.

- The elements that satisfy the predicate p are copied to the range beginning at d_first_true.
- The rest of the elements are copied to the range beginning at d_first_false.

2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If *first is not writable to d_first_true or d_first_false, the program is ill-formed.

Among the input range and the two output ranges, if any two ranges overlap, the behavior is undefined.

### Parameters

first, last | - | the range of elements to copy from
d_first_true | - | the beginning of the output range for the elements that satisfy p
d_first_false | - | the beginning of the output range for the elements that do not satisfy p
policy | - | the execution policy to use
p | - | unary predicate which returns ​true if the element should be placed in d_first_true. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt1, OutputIt2` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2, ForwardIt3` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

A std::pair constructed from the iterator to the end of the d_first_true range and the iterator to the end of the d_first_false range.

### Complexity

Exactly std::distance(first, last) applications of p.

For the overload (2), there may be a performance cost if `ForwardIt`'s value type is not CopyConstructible.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

partition_copy (1)
    template<class InputIt, class OutputIt1,
             class OutputIt2, class UnaryPred>
    constexpr //< since C++20
    std::pair<OutputIt1, OutputIt2>
        partition_copy(InputIt first, InputIt last,
                       OutputIt1 d_first_true, OutputIt2 d_first_false,
                       UnaryPred p)
    {
        for (; first != last; ++first)
        {
            if (p(*first))
            {
                *d_first_true = *first;
                ++d_first_true;
            }
            else
            {
                *d_first_false = *first;
                ++d_first_false;
            }
        }

        return std::pair<OutputIt1, OutputIt2>(d_first_true, d_first_false);
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <utility>

    void print(auto rem, const auto& v)
    {
        for (std::cout << rem; const auto& x : v)
            std::cout << x << ' ';
        std::cout << '\n';
    }

    int main()
    {
        int arr[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        int true_arr[5] = {0};
        int false_arr[5] = {0};

        std::partition_copy(std::begin(arr), std::end(arr),
                            std::begin(true_arr), std::begin(false_arr),
                            [](int i) { return 4 < i; });

        print("true_arr:  ", true_arr);
        print("false_arr: ", false_arr);
    }

Output:

    true_arr:  5 6 7 8 9
    false_arr: 0 1 2 3 4

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P0896R4 | C++11 C++17 | 1. the value type of `InputIt` (C++11)/`ForwardIt1` (C++17) was required to be CopyAssignable 2. the two output ranges could overlap | 1. not required 2. the behavior is undefined in this case

### See also

partition | divides a range of elements into two groups (function template)
stable_partition | divides elements into two groups while preserving their relative order (function template)
copycopy_if (C++11) | copies a range of elements to a new location (function template)
remove_copyremove_copy_if | copies a range of elements omitting those that satisfy specific criteria (function template)
ranges::partition_copy (C++20) | copies a range dividing the elements into two groups (algorithm function object)

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